Composite braiding is a filament placement operation wherein spools or cops wound with a composite filament material travel in a known manner circumferentially around a braiding ring (or braiding head) while depositing the material over a mandrel that moves axially relatively through the center of a braider guide ring which is positioned concentrically with the braiding ring. One half of the cops travel in a clockwise direction while the remaining half travel in a counterclockwise direction weaving alternately radially in and out of the first half. As the carriers move in a maypole-like fashion around the mandrel assembly, the mandrel moves axially through the center of the braider guide ring (or the braider guide ring moves axially back and forth over a fixed mandrel), thereby covering the mandrel with a braid of the composite filament.
The braiding machine was originally developed to produce many items that required continuous or repetitive braiding operations. Shoelace braiding is a typical example of such an operation wherein a continuous braid is formed and run until the cops or spools on which the filaments are wound are exhausted. Relatively few major changes have been made to the conventional braider since its inception, one of which was the changeover from vertical to horizontal braiding to facilitate braiding onto a mandrel. That change allowed a mandrel to be supported at opposite ends and reciprocated horizontally through the center of the braiding ring instead of vertically, thereby avoiding the cumbersome process of reciprocating a heavy mandrel vertically against the force of gravity. One such braider is depicted in FIG. 1, and a similar braider is disclosed in U.S. Pat. No. 4,519,290.
Although the change to horizontal braiding facilitated the manufacture of composite structures, composite braiding is still relatively cumbersome. Using a conventional braiding machine to braid composite materials over mandrels is time-consuming and relatively inefficient, and can make products fabricated with new high-technology composite materials undesirably expensive. A major element of the efficiency problem is the high ratio of braider set-up time to braider run time. That ratio and corresponding inefficiencies are high because a relatively long time is needed to fit the braider with replenished cops after each relatively short braiding run.
This set-up time problem is particularly acute in the manufacture of components for the aerospace field, where the braiding process must be stopped, the filaments cut, and all the cops replaced, before any of the cops run out of composite filament. This requirement exists to ensure that the full complement of filaments extends through the entire article, thus ensuring that the finished article meets strict strength specifications.
Where low weight is not critical, articles may be engineered with significantly higher factors of safety than is typical in aerospace applications. A higher safety factor in turn allows many non-aerospace articles to maintain adequate strength with less than the full complement of filaments. The safety factor designed into most non-aerospace articles thereby allows the braiding operation to continue after some of the cops have run out of filament and thus permits substantially all of the cops to be emptied.
The importance of maintaining a full complement of filaments is greater in the aerospace industry than in many non-aerospace applications because low article weight is a primary design requirement. Typically, low article weight is accompanied with tight factors of safety thus necessitating a requirement of high fiber fidelity to maintain adequate article safety margins.
Since low article weight is a priority, the aerospace industry does not typically enjoy the benefits associated with producing articles that have been designed with large safety factors. As a result, a full complement of filaments must be maintained, and to ensure that this result is attained, the braiding operation must be terminated while all the cops still have some unused filaments wound on them. Thus, when aerospace components are being fabricated, the problem of the high ratio of braider set-up time to braider run time is further aggravated since the amount of braided product that can be produced from each set of wound cops is substantially reduced.
After the cops are depleted, replacement of the cops on a large (144 carrier) braider can take from one and one-half to two hours. This process can be substantially longer than a braiding run which may typically be as short as 30 minutes. Thus, for every braiding run, three to four times the run time may be needed for set-up, resulting in inefficiencies which contribute to the costs of manufacturing composite products. These costs can make composite products less competitive in the marketplace than products fabricated from conventional materials.
Another problem with braiding composite products is that often, independent manual operations serving to further reinforce the article are required during the braiding process. Such operations frequently require that the braiding process be stopped and the mandrel withdrawn from the braiding ring after a layer of composite filaments has been applied. The manual operation is then performed on the mandrel while the braiding machine is idle. As a result, braider efficiency is again reduced due to down-time while manual operations are performed, thus contributing to the manufacturing costs associated with composite products.
A further problem which adds to the expense and inefficiency associated with composite braiding arises when it is desired to have two different sets of composite filaments braided over the mandrel. Many composite articles are composed of stacked layers of different material to achieve desired strength characteristics. Typical applications consist of alternate layers of carbon fiber filaments and glass filaments, or alternate layers of carbon fiber filaments and filaments of a material sold under the trademark Kevlar.RTM.. To effect this resultant layering, it is common practice to intermittently replace all the cops of one material with cops loaded with another material. Each changeover adds an additional one and one-half to two hours to the initial set-up time, further decreasing braider efficiency.